A flash analog-to-digital converter typically employs a set of voltage comparator circuits of which each has an analog signal input and a reference signal input. The analog signal inputs receive from a single output of a sample-and-hold circuit a DC voltage representing the amplitude of an analog voltage signal applied to the input of the sample-and-hold circuit and sampled at a particular time. The reference signal input of each comparator receives a different one of DC reference voltages developed by a series-connected resistor network. Each of the comparators provides an output voltage of changing amplitude that represents a first logic state (e.g., logic 1) whenever the sampled analog signal voltage is greater than the DC reference voltage and a second logic state (e.g., logic 0) whenever the sampled analog signal is less than the DC reference voltage.
The outputs of the comparators define collectively the bits of a digital output code. The comparators are arranged in an order such that successive outputs of comparators receiving DC reference voltages of increasing amplitudes represent successive output code bits of increasing order. This arrangement of comparators provides a digital output code, which is sometimes called a "thermometer" code, that nominally has a single transition between bits in the logic 1 state and bits in the logic 0 state. In a nominal thermometer code, the bits in the logic 1 state are of lower orders than those in the logic 0 state. An analog-to-digital converter of "N" binary output bits requires a thermometer code having 2.sup.N -1 bits and employs, therefore, 2.sup.N -1 comparators.
The voltage difference between adjacent DC reference voltages developed by the resistor network determines the quantization voltage or "LSB" value of the analog-to-digital converter. For example, an 8-bit analog-to-digital converter that is capable of processing an analog voltage signal with a peak-to-peak maximum swing of two volts has a quantization voltage or "LSB" value of about eight millivolts. Such a relatively small quantization voltage can make the analog-to-digital converter susceptible to errors in the thermometer code. Such errors, which typically result from voltage offsets in and different propagation delay times of the comparators, manifest themselves in a thermometer code that does not have a single transition between bits in the logic 1 state and bits in the logic 0 state. These errors produce an inaccurate thermometer code that provides an analog-to-digital converter output word which misrepresents the amplitude of the analog signal.